Explosive growth in the demand for analysis of samples in combinatorial chemistry, genomics, and proteomics is driving widespread efforts to increase throughput, increase accuracy, and to reduce volumes of reagents and samples required, as well as waste generated. Rapid developments in drug discovery and development are creating new demands on traditional analytical techniques. For example, combinatorial chemistry is often employed to discover new lead compounds, or to create variations of a lead compound. Combinatorial chemistry techniques can generate thousands or millions of compounds in combinatorial libraries within days or weeks. The generation of enormous amounts of genetic sequence data through new DNA sequencing methods in the field of genomics has allowed rapid identification of new targets for drug development efforts. There is therefore a critical need for rapid sequential analysis and identification of compounds that interact with a gene or gene product in order to identify potential drug candidates. Efficient proteomic screening methods are needed in order to obtain the pharmacokinetic profile of a drug early in the evaluation process, testing for cytotoxicity, specificity, and other pharmaceutical characteristics in high-throughput assays instead of in expensive animal testing and clinical trials. Testing such a large number of compounds for biological activity in a timely and efficient manner requires high-throughput screening methods that allow rapid evaluation of the characteristics of each candidate compound. Development of viable screening methods for these new targets will often depend on the availability of rapid separation and analysis techniques for analyzing the results of assays.
Microchip-based separation devices have been developed for rapid analysis of large numbers of samples. Compared to other conventional separation devices, these microchip-based separation devices have higher sample throughput, reduced sample and reagent consumption and reduced chemical waste. Liquid flow rate for microchip-based separation devices range from approximately 1-300 nanoliters (nL) per minute for most applications.
Examples of microchip-based separation devices include those for capillary electrophoresis (CE), capillary electrochromatography (CEC) and high-performance liquid chromatography (HPLC). See Harrison et al., Science 1993, 261, 895-897; Jacobsen et al., Anal. Chem. 1994, 66, 1114-1118; and Jacobsen et al., Anal. Chem. 1994, 66, 2369-2373. Such separation devices are capable of fast analyses and provide improved precision and reliability compared to other conventional analytical instruments.
He et al., Anal. Chem. 1998, 70, 3790-3797 describes the fabrication of chromatography columns on quartz wafers and reports an evaluation of column efficiency in the capillary electrochromatography (CEC) mode. The fabrication sequence described relies partly on standard, parallel microfabrication operations to create multiple separation channels and structures therein on which stationary phase materials may be coated. However, methods described for enclosing the separation channels as well as for providing fluidic access to and egress from the channels are decidedly non-standard and unsuitable for integration in a conventional, high-productivity microfabrication sequence.
Liquid chromatography (LC) is a well-established analytical method for separating components of a fluid for subsequent analysis and/or identification. Traditionally, liquid chromatography utilizes a separation column, such as a cylindrical tube, filled with tightly packed beads, gel or other appropriate particulate material to provide a large surface area. The large surface area facilitates fluid interactions with the particulate material, resulting in separation of components of the fluid as it passes through the separation column, or channel. The separated components may be analyzed spectroscopically or may be passed from the liquid chromatography column into other types of analytical instruments for analysis.
The separated product of such separation devices may be introduced as a liquid sample to a device that is used to produce electrospray ionization. The electrospray device may be interfaced to an atmospheric pressure ionization mass spectrometer (API-MS) for analysis of the electrosprayed fluid.
A schematic of an electrospray system 10 is shown in FIG. 1. An electrospray is produced when a sufficient electrical potential difference Vspray is applied between a conductive or partly conductive fluid exiting a capillary orifice and an electrode so as to generate a concentration of electric field lines emanating from the tip or end of a capillary 2 of an electrospray device. When a positive voltage Vspray is applied to the tip of the capillary relative to an extracting electrode 4, such as one provided at the ion-sampling orifice to the mass spectrometer, the electric field causes positively-charged ions in the fluid to migrate to the surface of the fluid at the tip of the capillary 2. When a negative voltage Vspray is applied to the tip of the capillary relative to the extracting electrode 4, such as one provided at the ion-sampling orifice to the mass spectrometer, the electric field causes negatively-charged ions in the fluid to migrate to the surface of the fluid at the tip of the capillary 2.
When the repulsion force of the solvated ions exceeds the surface tension of the fluid sample being electrosprayed, a volume of the fluid sample is pulled into the shape of a cone, known as a Taylor cone 6, which extends from the tip of the capillary 2. Small charged droplets 8 are formed from the tip of the Taylor cone 6, which are drawn toward the extracting electrode 4. This phenomenon has been described, for example, by Dole et al., J. Chem. Phys. 1968, 49, 2240 and Yamashita and Fenn, J. Phys. Chem. 1984, 88, 4451. The potential voltage required to initiate an electrospray is dependent on the surface tension of the solution as described by, for example, Smith, IEEE Trans. Ind. Appl. 1986, IA-22, 527≧535. Typically, the electric field is on the order of approximately 106 V/m. The physical size of the capillary determines the density of electric field lines necessary to induce electrospray.
The process of electrospray ionization at flow rates on the order of nanoliters per minute has been referred to as “nanoelectrospray.” Electrospray into the ion-sampling orifice of an API mass spectrometer produces a quantitative response from the mass spectrometer detector due to the analyte molecules present in the liquid flowing from the capillary. It is desirable to provide an electrospray ionization device for integration upstream with microchip-based separation devices and for integration downstream with API-MS instruments.
The development of miniaturized devices for chemical analysis—and, further, for synthesis and fluid manipulation—is motivated by the prospects of improved efficiency, reduced cost, and enhanced accuracy. Efficient, reliable manufacturing processes are a critical requirement for the cost-effective, high-volume production of devices that are targeted at high-volume, high-throughput test markets.
Attempts have been made to fabricate an electrospray device that produces nanoelectrospray. For example, Wilm and Mann, Anal. Chem. 1996, 68, 1-8 describes the process of electrospray from fused silica capillaries drawn to an inner diameter of 2-4 μm at flow rates of 20 nL/min. Specifically, a nanoelectrospray at 20 nL/min was achieved from a 2 μm inner diameter and 5 μm outer diameter pulled fused-silica capillary with 600-700 V at a distance of 114 2 mm from the ion-sampling orifice of an API mass spectrometer.
Ramsey et al., Anal. Chem. 1997, 69, 1174-1178 describes nanoelectrospray at 90 nL/min from the edge of a planar glass microchip with a closed separation channel 10 μm deep, 60 μm wide and 33 mm in length using electroosmotic flow. A voltage of 4.8 kV was applied to the fluid exiting the closed separation channel on the edge of the microchip to initiate electrospraying, with the edge of the chip at a distance of 3-5 mm from the ion-sampling orifice of an API mass spectrometer. Approximately 12 nL of the sample fluid collected at the edge of the chip before a Taylor cone formed and initiated a stable nanoelectrospray from the edge of the microchip. However, collection of approximately 12 nL of the sample fluid results in re-mixing of the fluid, thereby undoing the separation done in the separation channel. Re-mixing at the edge of the microchip causes band broadening, fundamentally limiting its applicability for nanoelectrospray-mass spectrometry for analyte detection. Thus, electrospraying from the edge of this microchip device after capillary electrophoresis or capillary electrochromatography separation is rendered impractical. Furthermore, because this device provides a flat surface, and thus a relatively small amount of physical asperity for the formation of the electrospray, the device requires an impracticably high voltage to initiate electrospray, due to poor field line concentration.
Xue et al., Anal. Chem. 1997, 69, 426-430 describes a stable nanoelectrospray from the edge of a planar glass microchip with a closed channel 25 μm deep, 60 μm wide and 35-50 mm in length. A potential of 4.2 kV was applied to the fluid exiting the closed separation channel on the edge of the microchip to initiate electrospraying, with the edge of the chip at a distance of 3-8 mm from the ion-sampling orifice of an API mass spectrometer. A syringe pump was utilized to deliver the sample fluid to the glass electrospray microchip at a flow rate between 100-200 nL/min. The edge of the glass microchip was treated with a hydrophobic coating to alleviate some of the difficulties associated with electrospraying from a flat surface and to thereby improve the stability of the nanoelectrospray. Electrospraying in this manner from a flat surface, however, again results in poor field line concentration and yields an inefficient electrospray.
In all of the devices described above, edge-spraying from a chip is a poorly controlled process due to the inability to rigorously and repeatably determine the physical form of the chip's edge. In another embodiment of edge-spraying, ejection nozzles, such as small segments of drawn capillaries, are separately and individually attached to the chip's edge. This process imposes space constraints in chip design and is inherently cost-inefficient and unreliable, making it unsuitable for manufacturing.
Desai et al., 1997 International Conference on Solid-State Sensors and Actuators, Chicago, Jun. 16-19, 1997, 927-930 describes a multi-step process to generate a nozzle on the edge of a silicon microchip 1-3 μm in diameter or width and 40 μm in length. A voltage of 4 kV was applied to the entire microchip at a distance of 0.25-0.4 mm from the ion-sampling orifice of an API mass spectrometer. This nanoelectrospray nozzle reduces the dead volume of the sample fluid. However, the extension of the nozzle from the edge of the microchip makes the nozzle susceptible to accidental breakage. Because a relatively high spray voltage was utilized and the nozzle was positioned in very close proximity to the mass spectrometer sampling orifice, a poor field line concentration and a low efficient electrospray were achieved.
Wang et al., 1999 IEEE International Conference on Micro Electro Mechanical Systems, Orlando, Jan. 17-21, 1999, 523-528 describes a polymer-based electrospray structure designed to spray from the edge of the chip, essentially replacing the mechanically fragile silicon nitride nozzle of Desai et al. with a polymeric nozzle. While the polymer substitution provides a significant improvement in mechanical reliability, additional non-standard processing materials and operations are required, making the fabrication of the structures incompatible with standard high-volume manufacturing facilities. Further, the presence of the polymeric material seriously limits the nature of subsequent processing operations and precludes high-temperature processing altogether. Concerns regarding sample contamination by monomeric residues in the polymer remain unresolved.
Thus, it is also desirable to provide an electrospray ionization device with controllable spraying and a method for producing such a device that is easily reproducible and manufacturable in high volumes.
U.S. patent application Ser. No. 09/156,037 (Moon et al.) describes electrospray ionization (ESI), liquid chromatography (LC), and integrated LC/ESI devices and systems and fabrication sequences to make them in silicon by reactive-ion etching. That application discloses methods of designing and fabricating those devices and similar ones in a manner that is consistent with well-established, cost-efficient, high-volume manufacturing operations. However, there are several aspects of the fabrication sequences and designs that potentially limit manufacturing yield. First, separation posts formed for purposes of liquid chromatography are subject to damaging mechanical stresses due to coating of additional films, wet immersions, and abrasion and clamping in the course of processing operations after formation of the separation posts. Second, etch lag in electrospray nozzle channels makes it difficult to complete the channel while controlling the height of the nozzle. Third, the formation of electrical contacts to the substrate in the presence of significant topographical steps of more than 1-2 μm is problematic due to an inability to uniformly and continuously coat photoresist for purposes of lithographic patterning and subsequent etching. Thus, improved processing operations and sequences are desired in order to ensure the high-yield manufacturability of such devices and systems. Further, such processing improvements that can be widely applied to a variety of MEMS and microfluidic devices and systems are highly desired.